The present invention is generally related to tape and disk storage systems and methods and, more particularly, to the field of partial response maximum likelihood (PRML) detection trellis based channels.
Traditional peak detection channels in magnetic disk drives have been replaced by partial response maximum likelihood (PRML) read channels to allow for higher recording densities where individual symbols written on the magnetic disk may overlap neighboring bit cells resulting in inter-symbol interference. Each bit decision made by a peak detection read channel is based on the read signal inside that bit cell only. The bit decision is completely independent to the read signal inside neighboring bit cells.
On the other hand, PRML read channels make use of the read signal inside a desired bit cell and neighboring bit cells to decide the content of the desired bit cell. Information from the neighboring bit cells results in improved (lower) bit error rates over conventional peak detection channels in the presence of additive noise. The lower bit error rate of PRML channels is usually traded for higher data densities. The data density is increased until the bit error rate degrades to the lowest acceptable level for a particular product.
PRML read channels use a complicated set of equations to make an optimal bit decision based upon the read signal within the desired bit cell and neighboring bit cells. PRML read channels became practical when Andrew J. Viterbi developed an algorithm that significantly reduced the number of computational steps required to make the optimal bit decision. Viterbi""s algorithm is generally called a xe2x80x9ctrellisxe2x80x9d and is represented by a two-dimensional graph of various allowable data patterns.
PRML read channels are relatively immune to additive Gaussian noise. However, PRML read channels are more susceptible than peak detection read channels to most other types of channel degradation such as amplitude loss, misequalization, and timing errors. Therefore, a PRML read channel produces lower bit error rates than a peak detection read channel when additive Gaussian noise is dominant, but higher bit error rates than a peak detection read channel when other types of signal degradation are dominant.
PRML read channels were adopted in disk drives before tape drives because disk drives have more additive Gaussian noise while other types of signal degradation are small. PRML read channels are now being used in tape drives. However, amplitude loss and other types of degradation common in tape signals reduce the potential PRML advantage.
Signal dropouts cause a large majority of most read errors in most tape drives. A signal dropout occurs when the amplitude of the read signal is lower than expected. In a tape drive, signal dropouts occur because of imperfect magnetic coating on the tape and contaminants contained on the tape. Other sources of signal degradation (such as additive noise or distortion) generally cause a much smaller minority of read errors in a tape drive. For a tape drive, the bit error rate of a PRML read channel inside a signal dropout is generally much higher than the bit error rate of a peak detection read channel (depending on the channel architecture and the dropout characteristics). Therefore, one of the greatest obstacles to using PRML read channels in a tape drive is their susceptibility to signal dropouts.
Accordingly, it is an object of the present invention to provide a partial response maximum likelihood (PRML) detection trellis method adaptable to signal dropouts.
It is another object of the present invention to provide a PRML detection trellis method adaptable to signal dropouts for use with a tape drive.
It is a further object of the present invention to provide a tape drive employing a PRML detection method adaptable to signal dropouts.
In carrying out the above objects and other objects, the present invention provides an adaptive trellis adaptable to read a signal having signal amplitude dropouts. The adaptive trellis includes a standard partial response maximum likelihood (PRML) trellis. The standard trellis is associated with a standard state diagram having standard branches between states. A first set of the standard branches produce a non-zero amplitude read signal sample and a second set of the standard branches produce a zero amplitude read signal sample.
The adaptive trellis further includes an estimating PRML trellis. The estimating trellis is associated with an estimating state diagram having estimating branches between states. The estimating branches produce estimated read signal samples that adapt to the amplitude of the signal.
The adaptive trellis further includes interconnections between the standard trellis and the estimating trellis. The interconnections allow a path to jump between a state of the standard trellis and a state of the estimating trellis as long as an interconnection between the two states is allowed by the standard trellis. A branch to the estimating trellis is not allowed if it is associated with a zero read signal sample in the standard trellis.
The adaptive trellis can also be represented by a single standard trellis with estimating branches added in parallel with the standard branches. The estimating branches are only placed in parallel with standard branches that produce non-zero read signal samples.
Each of the standard branches and the parallel branches have an error metric. The path with the lowest path error metric arriving at the first state is allowed to continue on and connect to the second state. The path error metric is the sum of all error metrics for all preceding branches that comprise the path. In an EPR4ML trellis, the error metric for a standard branch is (the amplitude of the read signal samplexe2x80x94target amplitude of the read signal sample){circumflex over ( )}2, where the target amplitude of the read signal sample=1.0, 0.5, 0, xe2x88x920.5, or xe2x88x921.0, assuming the monopulse peak amplitude is unity. The error metric for a parallel branch is (the amplitude of the read signal samplexe2x80x94target amplitude of the read signal sample*monopulse peak estimated by the path leading to the parallel branch){circumflex over ( )}2, where the target amplitude of the read signal sample=1.0, 0.5, 0, xe2x88x920.5, or xe2x88x921.0.
Further, in carrying out the above object and other objects, the present invention provides an adaptive trellis detection method adaptable to read a signal having signal amplitude dropouts.
The advantages associated with the present invention are numerous. The adaptive PRML detection channel in accordance with the present invention has been shown to have approximately the same dropout performance as conventional peak detection channels and approximately the same noise performance as conventional PRML detection channels in typical tape drive environments.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the present invention when taken in connection with the accompanying drawings.